The present invention relates to the field of control devices for automatic transmissions of automotive vehicles, and more particularly, relates to a transmission control device in which the characteristics of a conventional hydraulic fluid pressure control device are supplemented by an electrical override system.
There is a known form of automotive vehicle automatic transmission, comprising a fluid torque converter and a gear transmission mechanism equipped with a plurality of friction engaging means, the selective engagement of which provides various transmission speed stages. The selective engagement of these friction engagement means is, conventionally, automatically performed according to the operational condition of the vehicle in which the transmission is mounted, in order to control the gear transmission mechanism to be in a shift state which is most appropriate to the moment by moment operating conditions of the vehicle, by selective supply of hydraulic fluid control pressures provided by a hydraulic fluid pressure control device.
In such a hydraulic fluid pressure control device, there are conventionally mounted various shift valves which perform switching actions in dependence on a throttle hydraulic fluid pressure (throttle pressure) which is varied according to the amount an accelerator or throttle pedal of the vehicle is depressed, i.e., according to the amount of opening of the intake throttle of the internal combustion engine of the vehicle, and also in accordance with a governor hydraulic fluid pressure (governor pressure), which is varied according to the road speed of the vehicle. These shift valves select various shift stages for the gear transmission mechanism according to the relationship between this throttle pressure, or the amount the accelerator pedal of the vehicle is depressed, and the governor pressure, or the vehicle road speed.
However, the operational characteristics of such an automatic transmission are usually not entirely satisfactory. The functioning of the throttle hydraulic fluid pressure control valve which generates the throttle pressure is inevitably prone to a certain amount of hydraulic fluid leakage; and the same is true for the governor hydraulic fluid pressure control valve which generates the governor pressure. Further, since the line hydraulic fluid pressure (line pressure), which is a basic source of pressure which is controlled to produce the throttle pressure and the governor pressure, is inherently limited, due to the basic structure of the hydraulic fluid pressure control device, there exists a critical vehicle road speed, at road speeds over which the governor hydraulic fluid pressure valve, which typically modifies the line pressure to generate the governor pressure by using the balance between the centrifugal force on a valve member generated by the rotation of an output shaft of the automatic transmission and the governor pressure itself generated from the line hydraulic fluid pressure, can no longer generate a governor pressure which properly represents the road speed of the vehicle. Therefore, especially at the highest speed ranges of the automatic transmission, operating errors may become so large that they are troublesome, and, in the most extreme case, it may become impossible to provide good and satisfactory hydraulic fluid pressure control action.